The present invention is directed to the field of liquid scintillation counting.
Liquid scintillation counting is a method for measuring radioactivity of samples. The method is usually used to analyze beta activity and sometimes alpha, positron, or low energy gamma activity.
In usual practice, the samples of radioactive material to be analyzed are placed in vials containing a liquid scintillator. A vial containing the radioactive material, sometimes called the "test substance", and the liquid scintillator is referred to as a "test sample." The liquid scintillator converts the radioactive decay events emanating from the radioactive material in the test sample into light flashes having intensities corresponding to the energies of the decay events. The light flashes are converted by a photomultiplier tube into pulses corresponding to the intensities of the light flashes. The resulting pulses are applied to pulse height analyzers which only pass pulses falling in selected amplitude ranges, each analyzer being set to pass a different range of amplitudes. Each selected amplitude range is referred to as a "counting channel." The pulse rates in each counting channel are measured by the determination of the number of pulses occurring during predetermined time intervals. This process of analyzing test samples is referred to as "counting the test samples." The pulse rate determined provides an indication of the rate at which decay events are occurring in the test sample in the energy ranges corresponding to the amplitude ranges of the pulse height analyzer. This information can then be used to indicate the concentration of radioactive isotopes in the samples.
Because of chemical and light transmitting properties of the test samples, the intensity of the light flashes which reach the photomultiplier tube frequently do not correspond to the energy level of the decay events causing the light scintillation, but actually may be substantially reduced in intensity. This reduction in intensity in the light scintillations is a phenomenon known as "quenching." The amount of quenching, that is, the degree of intensity reduction, varies from test sample to test sample. The quenching results in the pulse rates detected in the different channels not being an actual reflection of the rate of decay events in the corresponding energy ranges.
The problem of quenching has been met by the use of various standardization methods. In these methods, the amount of quenching in each test sample is measured and the resulting measurement is utilized to convert the pulse rate detected in each channel to reflect the actual rate of occurrence of decay events in the corresponding energy ranges.
One commonly used method of standardization is referred to as the "internal standard method." This method involves counting the sample and then recounting after the addition of a known quantity of a standard radioactive material generally of the same isotope. The initial counting results are then adjusted by the use of calibrations derived from comparison of the added performance of the standard as measured with what said added performance should actually be.
Another standardization method is referred to as the "external standard method." In this method the sample is first counted alone. Then the sample is counted in the presence of an externally positioned gamma source which is the external standard. The pulse rate which would be caused in one of the channels of the system by the standard in a test sample without quenching is known. Accordingly, the degree that the pulse rate caused by the external standard in this channel is below the rate which the standard would cause in a sample with no quenching is an indication of the level of quenching in the test sample. Once this level of quenching in the test sample is determined, the first count can be adjusted to reflect the actual rate of occurrence of decay events in the corresponding energy ranges.
Another method of standardization is referred to as the "channel ratio method." In this method the ratio of counts in two dissimilarly set counting channels is determined. The sample is then evaluated by comparison of its ratio to standard curves obtained from known samples with varying degrees of quenching.
In the aforementioned methods, standardization techniques are utilized to provide calibrations whereby the actual rate of occurrence of decay events can be determined. Such actual rate can be determined more easily after the elimination of quenching effects. One method of eliminating or substantially reducing quenching effects is known as the "sample combustion method." This method is limited to materials which have volatile combustion products such as the isotopes .sup.3 H, .sup.14 C, .sup.32 P, and .sup.35 S.
In a typical sample combustion method, for example, as described in Belgian Pat. No. 715,254 or in French Pat. No. 1,573,284, the test substance is manually introduced into a combustion zone. The combustion products are collected in liquid scintillator in conventional counting vials. The vials are then manually brought to the automatic liquid scintillation counting system. This method has the disadvantage of including a number of manual steps.
A further advantage of the combustion technique is that certain volatile oxides are easily separated from one another by physical or chemical means, usually through use of cooling and/or selective absorbing agents. Such separation may permit the collection of samples originating from test substances containing more than one radioactive isotope, for example .sup.3 H and .sup.14 C, which samples have essentially only one of the two said isotopes, with almost no contamination from the other. Thereby, each sample may be counted under the best conditions for examining the contained isotope without the need for compromise operation to minimize the effect of the presence of the second isotope. Such complete isotope separation has not previously been attainable by the more conventional technique of dissolution or suspension of an unaltered or only slightly modified test substance in scintillator solution and counting.
It is an object of the present invention to provide a liquid scintillation counting technique utilizing a combustion step to overcome quenching effects and to permit chemical separation of .sup.3 H and .sup.14 C wherein manual steps are eliminated. A second object of the present invention is to eliminate the use of the conventional counting vial thereby resulting in substantially improved counting performance.